1. Field of the Invention
The present invention relates to a vibratory sensor and method of varying vibration in a vibratory sensor.
2. Statement of the Problem
Vibratory sensors, such as vibratory densitometers and vibratory viscometers, operate by detecting motion of a vibrating element that vibrates in the presence of a fluid to be characterized. Properties associated with the fluid, such as density, viscosity, temperature and the like, can be determined by processing a vibration signal or signals received from one or more motion transducers associated with the vibrating element. The vibration of the vibrating element is generally affected by the combined mass, stiffness and damping characteristics of the vibrating element in combination with the fluid.
The viscosity of a fluid can be measured by generating vibration responses at frequencies ω1 and ω2 that are above and below a resonant frequency ω0 of the combined fluid and vibratory sensor. At the resonance frequency ω0, the phase difference ϕ0 may be about 90 degrees. The two frequency points ω1 and ω2 are defined as the drive frequencies where the drive signal phase and the vibration signal phase differ by the phase differences ϕ1 and ϕ2, respectively. The phase difference ϕ1 may be defined as the point where the phase difference between the drive signal phase and the vibration signal phase is about 135 degrees. The phase difference ω2 may be defined as the point where the phase difference between the drive signal phase and the vibration signal phase is about 45 degrees.
The distance between these two frequency points ω1 and ω2 (i.e., the difference in frequency between ω1 and ω2) is used to determine the term Q, which is proportional to viscosity and can be approximated by the formula:viscosity≈Q=ω0/(ω2−ω1)   (1)
The resonant frequency ω0 is centered between the two frequency points ω1 and ω2. Therefore, the resonant frequency ω0 can be defined as:ω0≈0.5*(ω2+ω1)   (2)
The frequency points ω1 and ω2 are determined during operation when the sensor element interacts with the fluid to be characterized. In order to properly determine the frequency points ω1 and ω2, the prior art drive system uses a closed loop drive, driving the sensor element to alternate between the two phase difference points (ϕ1 and ϕ2) and recording the vibration frequencies ω1 and ω2 at these points. By using a closed-loop drive, the prior art drive system ensures that the phase difference measurement is stable when the vibration frequencies ω1 and ω2 are determined.
Alternatively, the frequency points ω1 and ω2 are defined as half-power points, as they comprise frequency points where the power in the vibration signal has half the power of the resonant frequency ω0, or where the half-power point amplitude Ahalf is (Ahalf=A0/√2). The A0 term is the amplitude of the vibration signal at the resonant frequency ω0. The two frequency points ω1 and ω2 are also known as 3 dB points, where the vibration signal power is 3 dB down from the resonant frequency power.
FIG. 1 shows a prior art vibratory sensor comprising a vibratory sensor element and a signal processor coupled to the sensor element. The prior art vibratory sensor includes a driver for vibrating a sensor element and a pickoff sensor that creates a vibration signal in response to the vibration. The vibration signal is sinusoidal in nature. The signal processor receives the vibration signal and processes the vibration signal to generate one or more fluid characteristics or fluid measurements. The signal processor determines both the frequency and the amplitude of the vibration signal. The frequency and amplitude of the vibration signal can be further processed to determine a density of the fluid, or can be processed to determined additional or other fluid characteristics, such as the viscosity.
The prior art signal processor generates a drive signal for the driver using a closed-loop drive circuit. The drive signal typically is based on the received vibration signal, wherein the prior art closed-loop drive circuit processes the received vibration signal to create the drive signal. The drive signal may be based on the frequency and amplitude of the received vibration signal, wherein the received vibration signal comprises feedback that enables the prior art drive system to achieve a target vibration. The prior art vibratory sensor drives the sensor element using the closed-loop drive and using a feedback element, wherein the closed-loop drive incrementally changes the drive frequency and monitors the feedback element until the desired target point is reached. The desired endpoint comprises a phase difference (ϕ) between the drive signal and the resulting pickoff signal achieving the phase difference ϕ1 or the phase difference ϕ2.
FIG. 2 is a flow chart of a method of operation of the prior art vibratory sensor for measuring fluid viscosity. Steps 1-4 below determine the frequency of the first frequency point ω1 while steps 5-8 determine the frequency of the second frequency point ω2.
In step 1, a vibration setpoint is set to a first target phase difference ϕ1 and the sensor element is vibrated from the current vibration frequency. The first target phase difference ϕ1 is achieved by varying the frequency of the drive signal, starting from the current vibration frequency. The current vibration frequency is gradually changed in a closed-loop manner and according to received feedback, such as feedback regarding the difference between a current phase difference and the target phase difference. The vibration frequency is incrementally ramped up or down from the current vibration frequency, depending on whether the phase difference is to be increased or decreased.
In step 2, the current phase difference is compared to the first target phase difference ϕ1. If the first target phase difference ϕ1 has been achieved, then the method proceeds to step 4. Otherwise, the method branches to step 3 until the first target phase difference ϕ1 is achieved.
In step 3, a wait is performed. Consequently, the method loops and waits until the vibration setpoint has been achieved. The prior art vibratory sensor therefore waits for the actual vibration of the sensor element to reach the vibration setpoint. Due to the closedloop drive operation, the sensor element does not achieve vibration at the vibration setpoint until at least a known wait time has elapsed.
The wait may be for a fixed predetermined time or may vary in length. Environmental conditions may require a longer than expected time to achieve the target phase difference. The length of the wait may depend on various factors. The length of the wait may depend on a distance to the target phase difference from the initial phase difference. The length of the wait may depend on the physical characteristics of the sensor element. The length of the wait may depend on the nature of the fluid being measured (including the density and/or viscosity of the fluid). The length of the wait may depend on the power available to the prior art vibratory sensor.
In step 4, where the vibration setpoint has been achieved and the phase difference between the drive sensor signal and the pickoff sensor signal corresponds to the first phase difference ϕ1, then the corresponding first vibration frequency ω1 is recorded. The first frequency point ω1 comprises the vibration frequency that generates the first target phase difference ϕ1. The first vibration frequency ω1 may comprise the frequency where the phase difference between the drive signal phase and the pickoff signal phase is about 135 degrees, for example.
In step 5, the vibration setpoint is set to a second target phase difference ϕ2 and the sensor element is vibrated from the current vibration frequency. The second target phase difference ϕ2 is achieved by varying the frequency of the drive signal, starting from the current vibration frequency. The current vibration frequency is gradually changed in a closed-loop manner and according to received feedback, such as feedback regarding the difference between a current phase difference and the target phase difference. The vibration frequency is incrementally ramped up or down from the current vibration frequency, depending on whether the phase difference is to be increased or decreased. It should be understood that the starting vibration frequency is therefore the current vibration frequency, which comprises the vibration frequency obtained in step 4 above.
In step 6, the current phase difference is compared to the second target phase difference ϕ2. If the second target phase difference ϕ2 has been achieved, then the method proceeds to step 8. Otherwise, the method branches to step 7 until the second target phase difference ϕ2 is achieved.
In step 7, a wait is performed. Consequently, the method loops and waits until the vibration setpoint has been achieved. Due to the closedloop drive operation, the sensor element does not achieve vibration at the vibration setpoint until at least a known wait time has elapsed, as previously discussed.
In step 8, where the vibration setpoint has been achieved and the phase difference between the drive sensor signal and the pickoff sensor signal corresponds to the second phase difference ϕ2, then the corresponding second frequency point ω2 is recorded. The second frequency point ω2 comprises the vibration frequency that generates the second target phase difference ϕ2. The second frequency point ω2 may comprise the frequency where the phase difference between the drive signal phase and the pickoff signal phase is about 45 degrees, for example.
FIG. 3 is a graph of a closed-loop vibration response of the prior art vibratory sensor of FIG. 1. The vertical axis represents vibration frequency (ω) and the horizontal axis represents time (t). It can be seen that the prior art vibratory sensor is alternatingly vibrated at the first frequency point ω1 and then at the second frequency point ω2, wherein this pattern is iteratively repeated. It should be understood that the first and second frequency points ω1 and ω2 are not necessarily constant. The first and second vibration frequencies ω1 and ω2 may change due to changes in the fluid being characterized by the vibratory sensor, for example.
Due to the closed-loop design of the drive portion of the prior art vibratory sensor, it can be seen that the actual vibration frequency changes smoothly and continuously, but slowly. Each change in drive frequency requires a closed-loop time period TCL to accomplish, due to the feedback used to achieve the target phase difference. As a result, the prior art vibratory tine sensor cannot measure rapid changes in ω1 and ω2, and therefore cannot measure rapid changes in viscosity of the fluid to be characterized. Further, even where the time period TCL is small, it can be seen that the time period TCL is repeated and will therefore add up and will affect the operation of the prior art vibratory sensor.